1. Field of the Invention
The invention relates generally to wellbore surveys. More particularly, the invention relates to the estimation of wellbore positions based on analytical techniques.
2. Background Art
Fluids, such as oil, gas and water, are commonly recovered from subterranean formations below the earth""s surface. Drilling rigs at the surface are often used to bore long, slender wellbores into the earth""s crust to the location of the subsurface fluid deposits to establish fluid communication with the surface through the drilled wellbore. The location of subsurface fluid deposits may not be located directly (vertically downward) below the drilling rig surface location. A wellbore that defines a path, which deviates from vertical to some laterally displaced location, is called a directional wellbore. Downhole drilling equipment may be used to directionally steer the wellbore to known or suspected fluid deposits using directional drilling techniques to laterally displace the borehole and create a directional wellbore.
The path of a wellbore, or its xe2x80x9ctrajectory,xe2x80x9d is made up of a series of positions at various points along the wellbore obtained by using known calculation methods. xe2x80x9cPosition,xe2x80x9d as the term is used herein, refers to an orthogonal Cartesian (x, y, z) spatial position, referenced to some vertical and/or horizontal datum (usually the well-head position and elevation reference). The position may also be obtained using inertial measurement techniques, or by using inclination and azimuth with known calculation methods. xe2x80x9cAzimuthxe2x80x9d may be considered, for present purposes, to be the directional angular heading, relative to a reference direction, such as North, at the position of measurement. xe2x80x9cInclinationxe2x80x9d may be considered, also for present purposes, to be the angular deviation from vertical of the borehole at the position of measurement.
Directional wellbores are drilled through earth formations along a selected trajectory. Many factors may combine to unpredictably influence the intended trajectory of a wellbore. It is desirable to accurately estimate the wellbore trajectory in order to guide the wellbore to its geological and/or positional objective. This makes it desirable to measure the inclination, azimuth and depth of the wellbore during wellbore operations to estimate whether the selected trajectory is being maintained.
The drilled trajectory of a wellbore is estimated by the use of a wellbore or directional survey. A wellbore survey is made up of a collection or xe2x80x9csetxe2x80x9d of survey-stations. A survey station is generated by taking measurements used for estimation of the position and/or wellbore orientation at a single position in the wellbore. The act of performing these measurements and generating the survey stations is termed xe2x80x9csurveying the wellbore.xe2x80x9d
Surveying of wellbores is commonly performed using downhole survey instruments. These instruments typically contain sets of orthogonal accelerometers, magnetometers and/or gyroscopes. These survey instruments are used to measure the direction and magnitude of the local gravitational, magnetic field and/or earth spin rate vectors respectively, herein referred to as xe2x80x9cearth""s vectorsxe2x80x9d. These measurements correspond to the instrument position and orientation in the wellbore, with respect to earth vectors. Wellbore position, inclination and/or azimuth may be estimated from the instrument""s measurements.
One or more survey stations may be generated using xe2x80x9cdiscretexe2x80x9d or xe2x80x9ccontinuousxe2x80x9dmeasurement modes. Generally, discrete or xe2x80x9cstaticxe2x80x9d wellbore surveys are performed by creating survey stations along the wellbore when drilling is stopped or interrupted to add additional joints or stands of drillpipe to the drillstring at the surface. Continuous wellbore surveys relate to thousands of measurements of the earth""s vectors and/or angular velocity of a downhole tool obtained for each wellbore segment using the survey instruments. Successive measurements of these vectors during drilling operations may be separated by only fractions of a second or thousandths of a meter and, in light of the relatively slow rate of change of the vectors in drilling a wellbore, these measurements are considered continuous for all practical analyses.
Known survey techniques as used herein encompass the utilization of a variety of means to estimate wellbore position, such as using sensors, magnetometers, accelerometers, gyroscopes, measurements of drill pipe length or wireline depth, Measurement While Drilling (xe2x80x9cMWDxe2x80x9d) tools, Logging While Drilling (xe2x80x9cLWDxe2x80x9d) tools, wireline tools, seismic data, and the like.
Surveying of a wellbore is often performed by inserting one or more survey instrument into a bottom-hole-assembly (xe2x80x9cBHAxe2x80x9d), and moving the BHA into or out of the wellbore. At selected intervals, usually about every 30 to 90 feet (10 to 30 meters), BHA, having the instrument therein, is stopped so that measurement can be made for the generation of a survey station. An additional measurement not performed by the survey instruments is the estimation of the along hole depth (measured depth xe2x80x9cMDxe2x80x9d) or wellbore distance between discrete survey stations. The MD corresponds to the length of joints or stands of drillpipe added at the surface down to the BHA survey station measurement position. The measurements of inclination and azimuth at each survey station along with the MD are then entered into any one of a number of well-known position calculation models to estimate the position of the survey station to further define the wellbore trajectory up to that survey station.
Existing wellbore survey computation techniques use various models, including the Tangential method, Balanced Tangential method, Average Angle method, Mercury method, Differential Equation method, cylindrical Radius of Curvature method and the Minimum Radius of Curvature method, to model the trajectory of the wellbore segments between survey stations.
Directional surveys may also be performed using wireline tools. Wireline tools are provided with one or more survey probes suspended by a cable and raised and lowered into and out of a wellbore. In such a system, the survey stations are generated in any of the previously mentions surveying modes to create the survey. Often wireline tools are used to survey wellbores after a drilling tool has drilled a wellbore and an MWD and/or LWD survey has been previously performed.
Uncertainty in the survey results from measurement uncertainty, as well as environmental factors. Measurement uncertainty may exist in any of the known survey techniques. For example, magnetic measuring techniques suffer from the inherent uncertainty in global magnetic models used to estimate declination at a specific site. Similarly, gravitational measuring techniques suffer from movement of the downhole tool and uncertainties in the accelerometers. Gyroscopic measuring techniques, for example, suffer from drift uncertainty. Depth measurements are also prone to uncertainties including mechanical stretch from gravitational forces and thermal expansion, for example.
Various considerations have brought about an ever-increasing need for more precise wellbore surveying techniques. More accurate survey information is necessary to ensure the avoidance of well collisions and the successful penetration of geological targets.
Surveying techniques have been utilized to estimate the wellbore position. For example, techniques have also been developed to estimate the position of wellbore instruments downhole. U.S. Pat. No. 6,026,914 to Adams et al. relates to a wellbore profiling system utilizing multiple pressure sensors to establish the elevation along the wellbore path. U.S. Pat. No. 4,454,756 to Sharp et al. relates to an inertial wellbore survey system, which utilizes multiple accelerometers, and gyros to serially send signals uphole. U.S. Pat. No. 6,302,204 B1 to Reimers et al. relates to a method of conducting subsurface seismic surveys from one or more wellbores from a plurality of downhole sensors. U.S. Pat. No. 5,646,611 to Dailey et al. relates to the use of two inclinometers in a drilling tool to estimate the inclination angle of the wellbore at the bit.
Other techniques have been developed to correct data based on measurement error. U.S. Pat. No. 6,179,067 B1 to Brooks relates to a method for correcting measurement errors during survey operations by correcting observed data to a model. U.S. Pat. No. 5,452,518 to DiPersio relates to a method of estimating wellbore azimuth by utilizing a plurality of estimates of the axial component of the measured magnetic field by emphasizing the better estimates and de-emphasizing poorer estimates to compensate for magnetic field biasing error.
There remains a need for techniques capable of utilizing overlapping survey data to better estimate the wellbore position and its related uncertainty of that position. Mathematical models have been used to estimate the wellbore position and position uncertainty in a wellbore. For example, SPE 56702 entitled xe2x80x9cAccuracy Prediction for Directional MWD,xe2x80x9d by Hugh S. Williamson ((copyright)1999), SPE 9223 entitled xe2x80x9cBorehole Position Uncertainty, Analysis of Measuring Methods and Derivation of Systematic Error Model,xe2x80x9d by Chris J. M. Wolff and John P. De Wardt ((copyright)1981), and xe2x80x9cAccuracy Prediction for Directional Measurement While Drilling,xe2x80x9d by H. S. Williamson, SPE Drill and Completion, Vol. 15, No. 4 Dec. 2000, the entire contents of which are hereby incorporated by reference, describe mathematical techniques used in wellbore position analysis. However, a specific position in a wellbore is often surveyed many times and by many different types of survey instruments at various stages of wellbore operations. Historically, these existing methods rely upon a sequence of non-overlapping surveys along the wellbore to estimate the position of a point in the wellbore, and fail to incorporate overlapping survey data.
It is desirable that overlapping surveys be taken into consideration when estimating positions in a wellbore. It is also desirable that a method of estimating positions in the wellbore, use overlapping surveys generated by downhole tools. The present invention provides a technique, which utilizes multiple overlapping surveys and combines the overlapping surveyed positions and related positional uncertainties of a given wellpath in order to produce a resultant wellbore position, or xe2x80x98Most Probable Positionxe2x80x99 (MPP), as well as an associated resultant positional uncertainty.
An aspect of the invention relates to a method for estimating a position in a wellbore. The method involves acquiring a plurality of surveys of the wellbore and combining overlapping portions of the surveys whereby the wellbore position is determined. Each measured survey defines a survey position in the wellbore and an uncertainty of the survey position.
Another aspect of the invention relates to a method for estimating a position in a wellbore. The method involves drilling a wellbore into a subterranean formation, acquiring a plurality of surveys of the wellbore and combining overlapping portions of the surveys whereby the wellbore position is determined. Each measured survey defines a survey position in the wellbore and an uncertainty of the survey position.
Another aspect of the invention relates to a method for estimating a position in a wellbore. The method involves taking a plurality of surveys of the wellbore and combining overlapping portions of the surveys whereby the wellbore position is determined. Each measured survey defines a survey position in the wellbore and an uncertainty of the survey position.
Another aspect of the invention relates to a method for estimating a position in a wellbore. The method involves acquiring a plurality of surveys of the wellbore and combining overlapping portions of the surveys whereby the wellbore position is determined. Each measured survey defines a survey position in the wellbore and an uncertainty of the survey position. The surveys are combined using the following equation: MPP=((HnTCovnxe2x88x921Hn)xe2x88x921HnTCovnxe2x88x921)*V.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.